Objectives: The aim of these studies has been the development of in vitro techniques for the investigation of the effects of environmental agents upon hematopoiesis. A tissue culture system that allows sustained proliferation and differentiation of bone marrow cells has been developed for the purpose of (1) identifying agents that are toxic to the marrow, (2) evaluating the role of metabolism in the action of various toxins, and (3) elucidating mechanisms of toxicity. The separation of bone marrow cells into morphologically homogeneous cell populations has been sought as an adjunct to the study of hematopoietic cells in culture. Successful isolation of individual cell types will facilitate the identification of cell-specific effects of environmental toxins and permit cell-cycle kinetic studies of bone marrow cells.